Spectral sensitizing techniques (that is, methods for expanding the light-sensitive wavelength regions of light-sensitive materials to the visible region and optionally further to the infrared region) are known. In this manner, the materials are made sensitive to light having a sufficiently long wavelength when photographic materials are prepared.
When spectral sensitizing dyes are added to photographic silver halide emulsions, the adsorption state thereof on the surface of silver halide crystals greatly varies, depending to a large extent on the addition conditions; in other words, changes in the conditions under which the dye is added to the emulsion affects absorption state. In turn, absorption state has a great influence on photographic effects such as fogging effect, sensitivity, spectral sensitivity distribution, desensitization and the stability of sensitivity. This is fully described in C.E.K. Mees, The Theory of the Photographic Process, Second Edition, chapter 12, pages 430-500 (Macmillan 1954).
Generally, most spectral sensitizing dyes for photographic silver halide emulsions are substantially insoluble in water. Hence, the spectral sensitizing dyes are usually added to the photographic silver halide emulsion by methods wherein the dyes are dissolved in organic solvents miscible with water, such as acetone, methanol, ethanol, propanol, methyl cellosolve and pyridine and then added to the emulsions. Optionally, a part of the organic solvent is replaced by water, and a mixture of the organic solvent and water is used. In addition, examples of methods for adding the spectral sensitizing dyes to the photographic silver halide emulsion include a method wherein a strong acid is added to a sensitizing dye having amidinium ion chromophore to protonate it and an aqueous solution thereof is added as described in JP-B-44-23389 (the term "JP-B" as used herein means an "examined Japanese patent publication"); and a method wherein a sensitizing dye which is substantially insoluble in water is mixed with a colloid of a volatile solvent and the mixture is heated to remove the solvent, thus dispersing the dye in the hydrophilic colloid as described in JP-B-44-22948.
However, these methods have various disadvantages. First, the methods wherein the dyes are dissolved in the organic solvents which are miscible with water and then the resulting solutions are added to the emulsion, have the following disadvantages. The organic solvents used cause a lowering in the surface activity of co-present coating aids, the coagulation of binders or the lumping of co-present couplers in the case of color photographic materials. This creates a difficulty in carrying out high-speed coating, such as coating at a rate of 100 m/min or higher. Further, when the solutions are added, the organic solvents used are immediately mixed with water so that there is a possibility that the sensitizing dyes are adsorbed by binders before they are adsorbed by silver halide crystals, or the dyes themselves may agglomerate together in which case only low-intensity spectral sensitivity can be imparted. In addition, the methods using the organic solvents are not preferred from the viewpoint of safety, in view of their volatility. Further, the method described in JP-B-44-23389 is not preferred from the viewpoint of the stability of the dyes, because the aqueous solution is made strongly acidic. The method wherein the dyes are dispersed in hydrophilic colloid as described in JP-B-44-22948 requires the use of organic solvents and hence has disadvantages described above in the case of the use of the organic solvents.
As alternative methods using no organic solvent, attempts have been made to add an aqueous dispersion of photographic additive to silver halide emulsions, the aqueous dispersion of photographic additive being obtained by dispersing photographic additive in the presence of a wetting agent or a dispersant in an aqueous solution. For example, the following methods are known.
JP-A-52-110012 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method wherein a sensitizing dye is crushed in the presence of a dispersant (surfactant) having a constant surface tension in an aqueous phase, water is removed from the resulting dispersion and the dye is dried and added as such to silver halide emulsions, or the dye is dispersed in water or an aqueous gelatin solution and the dispersion is added to silver halide emulsions.
JP-A-53-102733 discloses a method wherein a homogeneous mixture (pasty mixture) comprising photographic fine additive particles, a dispersant such as sorbitol and a binder such as gelatin is prepared and processed into noodle which is then dried with hot air, and the resulting powder is added to a photographic aqueous colloid coating composition.
U.S. Pat. No. 4,006,025 discloses a method wherein a spectral sensitizing agent is mixed with water to form a slurry, the temperature thereof is elevated to 40 to 50.degree. C., the slurry is homogenized or milled in the presence of a surfactant to uniformly disperse the spectral sensitizing agent in water and the resulting dispersion is added to silver halide emulsions.
Any of these methods is a method wherein photographic additives, such as spectral sensitizing agents, are dispersed in an aqueous system without using any organic solvent. However, these methods have the following practical problems. Since the aqueous dispersion is powdered by lyophilization, etc., the time required for the adsorption of the photographic additives such as the spectral sensitizing agent on silver halide is prolonged and hence the desired photographic sensitivity can not be obtained in a short time. Further, when such emulsions are coated, coating troubles are liable to be caused by precipitates, etc. Furthermore, since a wetting agent or a dispersant is used during the course of the dispersion of the photographic additives, this causes problems in that emulsions in the silver halide emulsions are broken, adverse effects are caused, for example, coating troubles are increased with the high-speed coating of silver halide emulsions and the adhesion of the resulting silver halide photographic materials is poor. These problems are obviously detrimental to products. Thereafter, an improved method described in JP-A-58-105141 was proposed. This method is a method wherein a water-insoluble compound is mechanically ground in an aqueous system at pH of 6 to 8 at a temperature of 60.degree. C. to 80.degree. C. to obtain finely divided particles having a particle size of up to 1 .mu.m and the resulting dispersion is added to a photographic emulsion. However, this method still has such problem that the desired photographic performance can not be obtained, even though considerable steps are taken to prepare the photographic material.
Recently, there has been proposed a method wherein cyclodextrin and ether derivatives thereof are co-present as means for forming an aqueous solution of a spectral sensitizing dye without using any organic solvent or strong acid as described in JP-A-62-215261. In this method, conventional spectral sensitizing dyes which are substantially insoluble in water are made slightly soluble in water to prepare photographic silver halide emulsions. However, expensive cyclodextrin derivatives are used and hence costs are increased. In addition, since cyclodextrin derivatives which are used may have deleterious effects on photographic performance (e.g., a lowering in sensitivity and the deterioration of the quality of layers), this method also has problems.
That the spectral sensitizing dyes which are substantially insoluble in water are solubilized and then added to the emulsion is a very significant problem in the preparation of photographic materials. Particularly, many cationic cyanine dyes have generally low solubility in water. The introduction of water-soluble groups, particularly dissociatable water-soluble groups (e.g., sulfo group, phosphoric acid group) into the molecules of these dyes has been often carried out to increase water solubility. Even when water solubility is improved, the desired photographic characteristics can not be obtained, because the properties of the compounds are varied. Accordingly, there is a substantial difficulty in adding the cationic cyanine dyes to aqueous systems.
Compounds belonging to the class of cationic cyanine dyes include dyes having 4-thiazolidinone ring, 5-thiazolidinone ring, 4-imidazolidinone ring, 5-inidazolidinone ring, 4-oxazolidinone ring, 5-oxazolidinone ring or 4-dithiolanone on a methine chain. These dyes are useful as spectral sensitizing agents for photographic silver halide emulsions and are often used in practice (see E. B. Knott, R. H. Jeffreys, J. Chem. Soc., 4762 (1952), ibid., 949 (1955)). These dyes can be used for spectral sensitization for a silver halide photographic light-sensitive material in the same manner as that of dyes as described in U.S. Pat. No. 3,674,499, JP-B-49-13331, JP-A-51-106425, JP-B-48-21564, Belgian Patent 532,028 and JP-A-54-18726. These dyes have very low solubility in water. Hence, use of these dyes leads to the above-mentioned problems, and these problems are unfavorable for the preparation of photographic materials.